New flash flood forecast and warning tools will soon become available in
all National Weather Service (NWS) forecast offices with the implementation of
the Flash Flood Monitoring and Prediction (FFMP) program version 2.0 (Smith et
al. 2000). FFMP computations of Average Basin Rainfall (ABR) are based
on the Areal Mean Basin Estimated Rainfall Program (AMBER) developed at the
Pittsburgh, PA NWS Office (Davis and Jendrowski 1996).

The new flash flood tools, developed by Paul Jendrowski (Jendrowski and
Davis 1998) are graphical enhancements of the original AMBER output of digital
ABR and ABR rate products. The enhanced AMBER software, created with Geographic
Information System (GIS) software called AMBERGIS. This paper shows how the AMBERGIS tools detected a rapidly developing
flash flood in Franklin, PA on 21 June 2001.

2.0 AMBERGIS STREAM DATABASE

A detailed stream database, created by the National Basin Delineation
(NBD) program (Cox et al. 2001) is used to create the graphic display of ABR and
ABR rate products. A sample of the NBD stream database is shown for the city of
Franklin, at the confluence of

French Creek
and the Allegheny River in Venango County in northwest PA (Fig. 1). Figure 2
shows the NBD stream segments that are included in the Pittsburgh NWS AMBERGIS
stream basin database for the Franklin area. The 11,180 defined segments are
assigned a unique identification number.

Each defined stream segment has a single outflow point and may have one
or more inflow

points. Two
small tributaries of the Allegheny River, segment 5624 (area 1.8 km2)
and

segment 5622
(area 3.3 km2) have been added locally to the NBD database. The

stream
segments provided by the NBD dataset have a minimum basin area of 5 km2.

The main rivers are divided into segments, providing complete hydrologic
connectivity. For example French Creek from Patchel Run to the Allegheny River
is segment 5632 (area 16 km2). Two segments of the Allegheny River
5623 (area 5.4 km2) and 5618 (area 5.1 km2) are shown. The
Allegheny flows into segment 5623 at Point A, flows into 5618 at point B, and
exits 5618 at point C.

Tributaries of the main rivers are also divided into stream segments.
Figure 3 shows how Patchel Run (total area 18 km2) is divided into
three stream segments.

The ABR and ABR rate for each stream segment are computed for each radar
volume scan (every 5 minutes). The ABR is stored in the real-time ABR database
in five-minute increments, and may be summed into any time increment. AMBERGIS
produces seven graphic products in the Digital Hybrid Scan Reflectivity (DHR)
polar one degree by one kilometer format including ABR rate and ABR
accumulations of 15, 30, 60, 90, 120 and 180 minutes. The time period for each
graphic is user selectable. A time lapse of the ABR Rate product can effectively
be used to show the persistent heavy rainfall over the same geographic area
(training of echoes).

Figure 4 shows the DHR polar grid plotted

on stream
segment 5632. Notice in Figure 2 that 60 mm of ABR fell in 5632. The AMBERGIS
graphic for 60 minute ABR would show all nine radar bins contained in 5632 with
a color code for 60 mm of ABR.

Fig. 4. DHR polar grid plotted on the French Creek stream segment (5632).
Center point of each bin is shown with a small “x”. Radar azimuth (degrees)
and range (km) are given for several bins. Major rivers are dark gray, while
urban areas are hatched light gray.

Heavy solid
lines are the segment boundaries.

4.0 FLASH FLOOD ASSESSMENT

The potential flash flood severity can be determined by comparing
observed ABR with flash flood guidance (FFG). The FFG is the amount of ABR
needed in a specific period of time to initiate flooding on a stream (Sweeney
1992). This county-based FFG is computed for time periods of 1,3,6,12 and 24
hours. The FFG computation assumes the stream is at low flow and no rainfall has
occurred in the basin since the rainfall data cutoff time. If streams are
running at high levels and/or multiple periods of rain have occurred in the
basin since the last ingest of rainfall into the FFG computation, the actual FFG
may be much lower that the posted FFG. Watershed
segments with a large percentage of urban coverage may have actual FFG
considerably below the county-based FFG. The FFG product also comes with the
disclaimer that “steep terrain can greatly reduce actual FFG”.

The amount of ABR over FFG is directly related to the severity of the
observed flash flooding. ABR of 25 mm over FFG may put 0.5 m of water on a
bridge, causing some cars to stall in the water. ABR of 50 mm over FFG in that
same watershed may result in 2.0 m of water on the bridge and wash cars off the
bridge and into the stream. When ABR equals FFG typically only minor flooding
problems occur as streams reach a bank full condition.

ABR must exceed FFG by a significant amount to produce serious flash
flooding. Based on years of ABR observations (1990 through 2001), the Pittsburgh
NWS office has found that ABR of 25 mm over FFG is often related to the start of
significant flash flooding, and ABR of 50 mm or more over FFG usually results in
serious flash flood occurrences. These locally observed thresholds may vary
considerably in other portions of the United States.

FFG for 21 June 2001 in Venango County, PA was 53 mm hr-1,
with significant flash flooding expected at about 78 mm hr-1. Notice
the observed ABR (Figs. 2,3) ranged from 31 mm to 65 mm, indicating that no
serious flash flooding should occur, but minor flash flooding might be expected.

Not all AMBERGIS stream segments have high flash flood potential. Larger
rivers serve as a natural boundary to flash floods. The volume of water
generated by the locally heavy rainfall producing flash flooding is usually
small compared to the volume of water needed to bring a large river to flood.
For example, the heavy rainfall near Franklin, PA on 21 June 2001 caused the
river gage on the Allegheny River at Franklin to rise from a stage (flow) of 1.0
m (90 m3 s-1) to 1.2 m (144 m3 s-1),
while flood stage at Franklin is 5.2 m (2,746 m3 s-1).

This is not to say that river segments are never subject to flash
flooding. The Saint Charles, PA river forecast point on Redbank Creek (area
1,403 km2) was struck with a flash flood on 19 July 1996 (Davis
2000). If heavy rainfall is spread over a large geographic area flash flooding
can occur on larger watersheds.

A stream segment type designation is used to remove stream or river
segments from the comparison of ABR vs. FFG. The stream segments types are
determined by the local NWS office and are not supplied with the NBD database.
Main river segments are usually removed from flash flood computations. Major river segments containing urban areas are included in the ABR/FFG
comparison. Flash flood rainfall can quickly overwhelm the storm drainage
network of urban areas causing severe urban flooding.

Most tributaries of the major rivers are typically flash flood streams,
such as Patchel Run in Figure 3. These streams are divided into stream segments,
as heavy rainfall and flash flooding may occur in small portion of the larger
stream watershed.

The ABR/FFG comparison is only valid if both the ABR radar estimate and
FFG are reasonable approximations of the actual ABR and FFG. ABR can be verified
by comparing radar estimates with observed rainfall (Davis 1997). Figure 5 shows
the radar estimate for the Franklin Airport rain gage was very close to the 56
mm measured at the gage site.

When AMBERGIS indicates the potential for flash flooding in a stream
segment, the stream name and possible damage track along the stream can be
inserted into the flash flood warning. During post-storm analysis, if the

flash flood
damage is accurately located, stream segments that contributed to the flash
flooding can be directly identified. Figure 6 shows the areas of flood damage in
the 3rd Ward north of French Creek and the length of Chubb Run along
the 15th Street hill. Stream flooding from Chubb Run (Fig. 7)
produced the flood damage along 15th Street. Chubb Run is contained
within the river forecast segment 5632, but should be locally subdivided as a
separate stream segment. No stream exists in the 3rd Ward, and
flooding in this area was the result of the rapid inundation of the urban storm
drainage system.

Two
factors combined to reduce the actual FFG in portions of segment 5632, the area
of urban coverage and the extreme slope of the topography. Figure 8 shows the
location of two cross-sections across the areas of flood damage. Davis (2001)
shows the typical slope (m/m x 1000) of “steep terrain” flash flood streams
range from 10 to 30 with some extreme slopes of 100-110. The Chubb Run

stream channel has a 146 m drop in 2800 m of reach for a
slope of 52. The
cross-section of the Chubb Run valley wall (Fig. 9) with a 150 m drop in a 1500
m reach has a slope of 100. The extreme slope of valley walls increases runoff
and creates a larger flood crest in the stream flow.

Fig. 9. Topographic cross section across Chubb Run from French Creek to
Galena Hill. Urban area of Franklin, PA is shown in dark gray.

Figure 10 shows the cross-section across the 3rd Ward. Grant
St through the 3rd Ward

(Route 322
in figure 6) was the center of the worst flooding north of French Creek.
Atlantic

St and
Pacific St parallel Grant St on the south and north respectively. Notice on the
cross-section that Grant St is several meters lower than the two neighboring
streets. When over

60 mm of
rain fell on the extreme slope of Oak Hill, a drop of 140 m in a distance of 800
for a slope of 175, Grant St was quickly inundated with chest deep water.

Fig. 10. Topographic cross section across the 3rd Ward from French Creek
to Oak Hill. Location of west-east roads along the cross section show with small
arrows.

6.0 ABR/ABR RATE PLOTS.

AMBERGIS graphic products are flash flood tools used to quickly zero in
on an area of potential flash flooding. Once a specific stream segment has been
located, AMBERGIS provides a line graph plot of the ABR and ABR rate for the
selected stream segment. From the line graph, the duration of the heavy rainfall
rates can quickly be determined along with the total ABR. Figure 11 shows that high ABR rates (>25 mm hr-1) started
at 2346 UTC and continued until 0026 UTC for a total of 45 minutes. ABR rates
remained above 75 mm

hr-1 from 2355 UTC to 0021 UTC, with 45 mm

of the 60 mm
total occurring in this 30-minute period.

While ABR rate is used to detect the developing flash flood, the
accumulated ABR determines when flooding begins. Increasing

values of
ABR above FFG result in more

serious
flash flooding. When the ABR total

reaches FFG,
minor flooding problems should begin, with the stream near bank full.

Additional ABR over FFG will bring the stream out of its banks. As the
ABR reached 50 mm with 1-hour FFG at 57 mm, a flash flood warning was issued
about 0025 UTC for Venango County, mentioning urban flooding possible in the
city of Franklin. Although only minor stream flooding should have occurred, the
heavy rainfall in an urban area prompted the issuance of the flash flood
warning. Some road flooding was reported by 0035 UTC with the urban flooding
reaching a peak around 0100 UTC.

The minimum basin area of the defined watersheds is critical to the
detection of flash flooding. Stream segments must be small enough to detect the
heavy rainfall responsible for flash flooding. Rainfall gradients associated
with thunderstorm are extreme. Figure 12 shows the ABR/ABR rate line graph for
the three combined watersheds segments that make up Patchel Run. This watershed,
just 2 km northwest of Franklin, received only 35 mm of ABR with no stream
flooding observed in the watershed. A minimum basin area of 5 km2 is
required to detect these important spatial variations in ABR.

ABR and ABR rate have proved to be important new flash flood tools for
the early

detection of
flash flooding. Graphic displays of the ABR data allow quicker access to the
large and detailed rainfall database. The ABR rate can be used to monitor
developing flash floods before flooding begins, increasing warning

lead time.
Availability of the small stream

database
allows the inclusion of stream specific data in warnings and statements.
Comparisons of ABR with FFG can provide guidance on the initiation of flooding
the potential severity of the flash flooding. These flash floods tools should
greatly enhance the flash flood program of the NWS as the FFMP software becomes
operational in 2002.